StuartR wrote: - much less understood.
Communication usually fails, except by accident.
If a message can be interpreted in several ways, it will be interpreted in a manner that maximizes the damage.
There is always someone who knows better than you what you meant with your message.
Korpela's First Corollary: If nobody barks at you, your message did not get through.
I agree with what has been said early on in the thread; I don't see these receipts as stable, they didn't work well in the "early" days (I've seen request for delivery receipt being interpreted as request for read receipt), and today I would expect most to ignore them (and more, such as linked images that can tell if and when an email has been opened, how often (if image isn't cached), and if it has been forwarded, sort of). If you ask me HTML has almost zero place in email.
One can't force people to do anything, and we are all different (some even respond to obvious spam (spam being one of the reasons why these requests are ignored)). If there is a working relation underneath (teenagers is a special case), and if there is something to respond to, then there usually will be some response, at some point.
Byelingual When you speak two languages but start losing vocabulary in both of them.